Type C viruses have been causally linked to leukemia and sarcoma in a variety of vertebrate species including at least one primate. However, the role of these viruses in human malignancy remains unclear. We have conducted experimentation in several areas to further elucidate this problem. A wide range of human tumor specimens have been analyzed for the presence of infectious type C virus by techniques of in vitro cocultivation, thus far with negative results. Moreover, many of the same tissues were analyzed by nucleic acid hybridization for the presence of gene sequences related to those of the endogenous type C virus of baboons, the only endogenous virus isolated from primates. There is, at present, no firm evidence that this virus has been horizontally transmitted to man, thereby resulting in malignancy. Finally, a series of experiments were conducted to determine whether antibody-independent complement-mediated viral lysis might prevent the spread or release of type C viruses in primates. However, such virolytic activity was detected in those primate species which do, in fact, release and horizontally transmit type C viruses.